ALACPA-ICAO Seminar on PMS

Lima Peru, 19-22 November 2003

Airport Pavements FWD/HWD Testing and Evaluation

By: Frank B. HoltVice President

Dynatest International A/S

Dynamic Testing

• The method of FWD/HWD testing simulates real load conditions

Not to be confused with dynamic analysis

Dynamic Testing• Testing performed on

– Highways– Airfields– Construction sub-base

• Output data used– Strengthening and Maintenance– Pavement Management System– New Design– Airfield parameters– Quality Testing

Load Distribution

Load

Strong Pavement

Load

Weak Pavement

Subgrade

Base

Surface

Theory of Elasticity

• Analytical-empirical method– Calculation of pavement response

• Layered system• Critical stresses, strains or deflections• Loading

• Most widespread method used– Two material parameters needed

• Young’s modulus and Poisson’s Ratio

– Hooke’s law• Ratio of stress over strain is constant• Ratio radial over longitudinal strain = Poisson’s Ratio

Load – Stress - Strain

L

∆L/2

∆D/2

D

Sample in unloaded condition

Sample in loaded condition

µ = εD/εL

εD =∆D/D

εL=∆L/L

Area A

Load Q

σ = Q/Α

Elastic Modulus

STRESS

STRAIN

ELASTIC RANGE

STRENGTH

E = σ/ε

Linear Elastic System• ASSUMPTIONS

• LINEAR ELASTIC• HOMOGENEOUS• ISOTROPIC• CONTINUOUS (HORIZONTAL)• HALF SPACE (VERTICAL)

• INPUTS• LOAD• LAYER THICKNESS• LAYER MODULUS• POISSON’S RATIO

Layered System

p - contact pressure

P

Surface E1 , µ1

Base E2 , µ2

Subgrade E3 , µ3

h1

h2

α

Radius r or aTotalLoad

Typical Modulus Values

30-300Subbase

100-1000Granular Sub-base

20000-30000PQ Concrete

8000-15000Lean Concrete

3000-7000Bituminous @200C

Modulus (MPa)Material

Typical Poisson Ratio Values

0.45Soils (fine-grained)

0.4Crushed Stone

0.2Cement Bound

0.35Bituminous Bound

Poisson’s RatioMaterial

Why use a FWD/HWD?• In order to determine layer moduli for

analytical design testing equipment must:– simulate loads similar in magnitude to the

actual loads experienced by the pavement– measure loads to very high degree of

accuracy– measure deflections to a high degree of

accuracy at large radial distance from the load (deflection bowl)

Structural Condition

• HWD survey vital structural component– allows proactive measures– reliable input required

• layer thicknesses• mechanistic models

HWD Approach & Analysis

• Minimum center deflection of 150 microns

• GPR linked input for analysis• Core borings for calibration• Point by point analysis• Linear and non-linear approach• Normal distribution concepts for lateral

wander

Structural Evaluation

• FWD/HWD allows non-destructive testing of pavements

• Detects strength/weakness of all layers

• Enables detection of weakness prior to surface failure

Analytical Pavement Evaluation

1. Back-calculation of deflection bowl

2. Determine pavement life

3. Determine maintenance requirements

Response Models used for Back-calculation

• Radius of Curvature• Method of Equivalent Thicknesses

(MET)– Easy Simplified Model using a Linear

Elastic Model • Layered Elastic Model (LEM)

– Linear subgrade• Finite Element Model (FEM)

Common Back-calculation Software

• Modulus (LET)– American Standard Units– Metric Modulus (no forward calculation)

• ELMOD 5– curvature– FEM/LET/MET models

• PADAL/BISAR/PCASE

Back-calculation

• Input– Material Properties– Layer Thicknesses– Applied Load

• Output– Deflections

• Input – Measured Deflections– Applied Load– Layer thicknesses

• Output– Layer stiffnesses– Calculated Deflections

and %error

Forward Calculation Back-calculation

Response Locations

CL

4 - COMP. STRAIN

3 - COMP. STRAIN2 - TENSILE STRAIN

1 - DEFLECTIONSURFACE

BASE

SUBGRADE

Residual Life and Overlay design

• Inputs– Strains and stresses– Load– Fatigue curves

• Asphalt Strain Criteria – Bottom–up Cracking

• Concrete Stress Criteria – Cracking

• Subgrade Strain Criteria – Permanent Deformation

• Output – No. of load repetitions until failure

Fatigue Curves

• Asphalt strain at the bottom of the layer

• Concrete stress at the bottom of the layer

• CTB stress at the bottom of this layer• Subgrade strain at the top of the

layer

Ullidtz: Pavement Analysis

§ εt = K * (Nf/106)(-1/a) * (E/Eref)b

• ε t = allowable horizontal tensile strain

• Nf = load repetitions to failure

• E = asphalt modulus

• Eref = reference modulus

• K, a, b = material constants

• b = often zero (0)

ELMOD 5

§ Strain = A * (N/106)B * (E/Eref)C

or§ Permissible value = A * Mload

B * (E/Eref)C

• A = µstrain• Mload = N * 106

• C = 0

Parameter Screen

Asphalt Reference Materials

03000-0.200225NAASRA

03000-0.178195DK (Kirk)

-0.2593000-0.304240AI (Ullidtz)

-0.2596.9-0.3041162AI

03000-0.250538SHELL

03000-0.240251TRL (DBM)

03000-0.231224TRL (HRA)CErefBAReference

Fatigue CurvesASPHALT FATIGUE

10

100

1000

10000

1000 10000 100000 1000000 10000000

LOAD REPETITIONS (N)

TE

NS

ILE

ST

RA

IN (

10^6

)

E = 3450 MPa (500ksi) E = 1380 MPa (200 ksi)

Unbound materials

Reference A ustrain B constant Eref C constant

TRL&Nottingham 451 -0.280 160 0

SHELL 885 -0.250 160 0

Asphalt Institute 484 -0.223 160 0

Reference A B constant Eref C constant

Asphalt Institute 0.1425 -0.307 160 1.16 & 1

Denmark (Kirk) 0.12 -0.307 160 1.16

Vertical strain

Vertical stress

Seasonal Adjustments

• Seasonal variations defined in Parameter Setup file– how is the yearly temperature variation?– how is the yearly variation in unbound

material• Elmod features

– define up to 12 seasons– define climatic constants for each material– Define asphalt modulus/temperature

relationship

Seasonal Adjustments

• Seasonal constants can be:– entered manually

or

– calculated automatically according to user defined sinouisdal or exponential curves

Asphalt Temperature Variation

0

0.5

1

1.5

2

2.5

3

3.5

-10 0 10 20 30 40

Temperature (Celsius)

Stif

fnes

s F

acto

r

Concrete Pavements

Joint Analysis

• Westergaard Theory• Inputs

– FWD Setup changes– As for OB Theory– Joint location

• Outputs– Equivalent foundation stiffness (k-value)– Void Intercept– Joint Condition– Load Transfer– Support conditions

Geophone SetupWhen testing at a joint (or corner) the geophones at distances 8 in. (200 mm) and 12 in.(300 mm) from the load centre must be placed on either side of the joint, as shown below:

Concrete Temperature Variation

• Warping of slabs– Temperature gradient

• Joint expansion• Summary

– Night time slab centre testing– Daytime load transfer testing

Design Loads

• Design loads defined in Parameter Setup file

• Elmod can handle a mix of up to 12 different loads

• Usually for roads all traffic is converted into 1 design load

• For airfield it is advised to base the calculation on a mix of different aircraft types

Design Loads

• A design load is defined by:– wheel load– tire pressure– wheel and axle configuration– percentage of total traffic

Miner’s Law

• THE PRINCIPAL OF LINEAR SUMMATION OF DAMAGE– IF LOAD A FATIGUE LIFE IS NfA AND LOAD B IS NfB THEN

DAMAGE DUE TO 1 PASS OF EACH LOAD IS

• GENERALLY , THIS CAN BE WRITTEN AS

D = 1 N 1 Nf fA fB+

D = (Fatigue)

D = (Rutting)

f

f

1

1

N

N

fii

rii

∑∑

Overlay Design

Calculatestress/strain

Calculateallowable

traffic

Relate to residual life

Does residual life match

design life ?

Adjustoverlay

thickness

Overlay design

Yes

No

ELMOD 5• Output example of responses

ELMOD 5• Output example Life & Overlay

ACN/PCN Method

• ACN– Aircraft Classification Number

• PCN– Pavement Classification Number

PCN according to ICAO

• PCN: Pavement Classification Number

• ICAO: International Civil Aviation Organization

• “A number expressing the bearing strength of a pavement for unrestricted operations”

• Any method may be used

ACN according to ICAO

• ACN: Aircraft Classification Number• “mathematically derived single wheel

load to define the landing gear/pavement interaction”

• ACN = ESWL * 2/1000 kg• Flexible: ACN = f(CBR)• Rigid: ACN = f(k-value)

Rigid pavement ACN

• Reporting stress = 2.75 MPa• Calculate thickness of concrete for

actual gear (to produce 2.75 MPa at bottom)

• Calculate ESWL (1.25 MPa tire pressure) to give same stress with same thickness

Flexible pavement ACN?

• Calculate “t” to give same deflection at subgrade from actual gear and ESWL

• Multiply “t” by “load repetition factor”? (0.9 dual, 0.825 dual tandem)

• Recalculate ESWL from equation

035.325692.0ESWL

CBRESWLt −=

Calculation of PCN

• Moduli are derived from FWD testing (using Elmod3 approach)

• Moduli are modified for seasonal effects• The ESWL, which match the fatigue

relation for the “unrestricted usage” number, is calculated

• Rigid: stress in concrete only• Flexible: stress on subgrade only

Thank You!